Lance Bradstreet

Lance Bradstreet John McIntyre Project Meteor 10/8/06 RIT Project METEOR RITSAT1 Communications Subsystem

Overview Due to the high risk environment in which the satellite will operate, a dual band parallel path communications scheme will be used to provide a requisite amount of redundancy. Two dual band transceivers will be switched between two single band antennas allowing for cross band full duplex operation or half duplex on any one radio in either or both of the bands. In all, in the event of a failure or link problem, there are four distinct radio configurations that can be used.

Transceivers The backbone of the communications system is comprised of two Alinco DJ-C7T/E VHF/UHF wideband FM transceivers. Each radio can be used as the primary transmitter or receiver. When both radios are operational we will have the capability of having one radio continuously transmitting and the other continuously in receive mode, waiting for ground station commands. In the case of a single radio failure, the remaining radio will operate in half duplex mode. In the event of a single antenna failure or alignment or polarization problem half duplex could be continued on a single channel on the remaining antenna as well. The redundancy of two radios decreases the chance of a communication failure.

Channel Selection The satellite will receive data on a UHF channel and will transmit data on a VHF channel. This arrangement was chosen because our most marginal path is the satellite downlink due to the low power constraints. The atmospheric path loss is less in the VHF band. The extra path loss on the UHF side can be recovered by increasing the ground station transmitter power. The amateur satellite frequency bands are allocated at 144- 146MHz and 435-438MHz. The radios selected can transmit between 300-500mW of power in these bands. The receiver radio also has a sensitivity of about -120dBm for 12 dB SINAD. The amateur frequency bands were selected over the ISM or other commercial or experimental license bands due to the pre-existing network of satellite ready amateur radio operators who are willing and able to assist in recovering the signal from RITSAT1 during the times in the satellite’s orbit when it is not over our ground station, or in the event of a ground station failure. There is also a pre-existing knowledge base and mechanism for licensing and flight approval available within the amateur radio community.

Modulation Scheme In keeping with the amateur radio and OSCAR standards, the Bell 202 FSK modulation scheme was selected. This has been used by previous amateur satellites and there is an appreciable number of other stations capable demodulating this signal. There are also lightweight and low power modems available that will easily interface with our transceivers for this standard. To go to a higher data rate, say 9600 BPS with FM transceivers would exceed the limitation of the audio bandwidth and would require a Lance Bradstreet John McIntyre Project Meteor 10/8/06 direct frequency modulation modem which would increase the complexity and size of our communications system and limit the number of available ground stations capable of demodulating our downlink signal.

The modem selected for use is the MX614 Bell 202 FSK Modem. This is a cheap, low power modem which converts data signals to audio tones. The modem can be operated with a 3.3V supply at 1mA of current. The modem is capable of 1200 baud as well as slower data rates. With a 12 dB S/N ratio, the modem will provide a 10-6 bit error rate. Other modems had higher data rates but were larger and used more power. The MX614 was the smallest and most low power modem found.

Gain Stages RF Paths Between each transceiver and the antenna switch there will be three selectable paths. For when the DC power budget allows, there will be a power amplifier providing at least 1-2 Watts of power to the downlink antenna that can be selected if necessary. This amplifier will max out the power budget and will deplete the batteries very quickly when used, but in the event of an unforeseen path loss or interference, it may afford some telemetry at a lower duty cycle and not leave the satellite totally without downlink ability. For when the transceiver is selected as the receiver, there will be a low noise amplifier path which will allow for greater uplink signal recovery. For low power transmit, or in the event of an amplifier failure, there will be a unity gain bypass path that can be selected as well.

Antennas There will be two antennas on the satellite on opposing sides of the structure that should be oriented normal to the earth’s surface as a result of our passive attitude control. During rocket flight, the VHF downlink antenna will be needed to send rocket telemetry data, so it will remain deployed, sticking through the center of the nose cone. The other uplink antenna will either stick down into the rocket or bend over and deploy when the satellite is ejected from the rocket. To keep the length of the uplink antenna short so that it does not need a large pocket inside the rocket, a single band UHF quarter wave antenna will be used. The transmit antenna would then be a single band VHF quarter wave antenna on the opposite face. While using dual band antennas would allow us to select the antenna with the best orientation for downlink, dual band antennas suffer matching and loss problems and are consequently very inefficient for a low power system. The satellites antennas will be omni-directional and linearly polarized. So that orientation is does not have to be considered in the path and attitude control becomes less critical, circularly polarized antennas will be used on the ground. There will be a polarization loss due to this, but it is more than made up for in ease of antenna alignment and lengthening of the communications window. This configuration has been used and tested by other small satellites successfully. To provide a sufficient ground plane for the antennas, ground radials will be attached to the four remaining sides without antennas. They will be comparable to the length of the VHF antenna and extend in a plane orthogonal to both antennas. These will also be left deployed during flight to provide a ground plane for the antenna when it is transmitting during rocket flight. Lance Bradstreet John McIntyre Project Meteor 10/8/06 Link Path The full link budget can be seen in the link budget table and diagrams. The most marginal point of the link is when the satellite is just entering and exiting the window. All of the link budget calculations were done for a “worst case” of when the satellite is farthest away in a 10 minute window passing directly overhead. The path that is most susceptible to degradation and loss is the downlink path because the transmitter is very low power. To recover the signal, a high gain, > 9 dB, antenna on the ground must be used as well as cascaded LNA’s with extremely low noise figures, < 1 dB. The uplink path will not be as critical because we have nearly unlimited power budgets on the ground for the transmit station. The FCC restricts our ground transmitters to 1500W. Realistically, a transmitter and amplifier giving 50 – 150 W would be easily acquired and sufficient for uplink.

Other Configurations Considered Other design ideas included using small data radio modules that can be crystalled or programmed into the ham bands with a power amplifier instead of an actual handheld ham radio. This idea was originally pursued because of the potential for increased efficiency. The radios were lightweight and low power; they also did the job of the modem. These radios were not used because they didn’t offer a selection of frequencies. The frequency coordination process will take a great deal of time and our schedule for a completed satellite will not allow for turnaround of a custom radio after we get frequencies coordinated. Also, each subsequent launch may be coordinated on different frequencies, requiring a crystalling or returning of the radios. The Alinco radios are VCO tuned in 125 KHz steps anywhere inside the amateur satellite bands and can be reprogrammed at any time through software.

Alternative configurations could use a single transceiver or a dedicated transmitter and receiver. Using a single transceiver removes the possibility of full duplex operation. Dedicated transmitters and receivers leave the satellite more vulnerable to failure. If one of the two transceivers fails, half duplex mode can still be accomplished.

There exist many low power and high speed data radios in the ISM and commercial bands that were seriously considered for this application. They were not selected because the advantages of operating within the ham bands were too great. There are ground stations world round who are capable or recovering our data when the satellite is out of range of the RIT ground station.

Primary Communications Risks and Concerns There are many risks taken with a low power satellite and consequently there are multiple points of potential failure within the communications system. When the satellite is ejected from the rocket, if it catches on release and tumbles in its orbit, the communications link could be intermittent. This risk has been mitigated by omni- directional antennas. An extreme physical environment and exposure to radiation leave the communications system aboard the satellite very vulnerable to device failure. The modem, transceivers, RF switches and amplifiers are all vulnerable to radiation. To mitigate this risk, redundancy has been included with the two transceivers and unity gain Lance Bradstreet John McIntyre Project Meteor 10/8/06 bypass paths. The rocket will not be actively controlled, so there is risk of an error in the guidance and we are deployed into an orbit which was not predicted and the signal path is consequently higher loss. This risk has been mitigated by including a power amplifier that can be turned on if necessary, though at the cost of the DC power budget and transmit duty cycle. If the satellite’s orbit does not come near enough to RIT’s ground station for signal recovery, the use of the amateur radio bands means that there are numerous other ground stations around the globe that can assist in data recovery.